Obesity is a health concern in horses worldwide, with a prevalence estimated between 31 – 45% in some equine populations. [1]

Among ponies and easy keeper breeds, the prevalence is even higher with one study reporting that 72% of adult ponies were overweight or obese. [2]

Horses are generally considered obese when they have a body condition score of greater than 7 on the Henneke 9-point scoring system. [1]

Obesity is a welfare concern for horses because it increases the risk of other health conditions including arthritis, respiratory problems and poor fertility. Obesity may be a sign of metabolic syndrome, which predisposes horses to laminitis. It can also negatively impact your horse’s performance and soundness. [3]

While many factors can influence obesity, horses become overweight when they consume more energy than they expend on activity and maintenance over a long period of time. [4]

In this article, we’ll explore the causes, consequences, and risk factors for obesity in horses. If your horse is obese, speak with your veterinarian and your equine nutritionist to formulate a plan for healthy weight loss.

Is my Horse Obese?

Obesity is defined as the excessive accumulation of body fat resulting in a negative impact on the horse’s health. [1]

A horse is considered obese when fat accounts for more than 20% of its total bodyweight. Some very obese horses may have a body fat percentage of up to 40%. [1]

Judging whether your horse is overweight or obese can be challenging; research shows that horse owners frequently underestimate how fleshy their equine companions are. [5]

The 9-point Henneke Body Condition Scoring system can be used to evaluate whether your horse is overweight or obese.

This scale assesses your horse’s body condition by observing and palpating specific areas where fat accumulates in the body. These areas include the crest of the neck, the withers, behind the shoulders, over the rib cage, on the rump, and above the tailhead. [6]

Equine Body Condition Scoring

Horses that score 4, 5, or 6 are considered to be a healthy body weight. [6] Generally, horses with a body condition score of 7 or higher are considered obese. [7]

You can determine your horse’s body condition by printing out Mad Barn’s Equine Body Condition Score Worksheet.

Body Condition Scoring for Horses

Risk Factors for Equine Obesity

Any horse can become overweight or obese; however, some horses are more prone to gaining weight than others.

Factors that affect a horse’s risk of gaining too much weight include: [8][9][10][11]

  • Feed provision
  • Breed & Genetics
  • Dominance
  • Age
  • Workload
  • Management
  • Season
  • Injury or stall rest

Feed Provision

Providing more calories than the horse needs over a long period leads to fat accumulation, weight gain and obesity.

Overfeeding results in a positive energy balance and can occur if your horse:

  • Is fed too much feed
  • Is consuming energy-dense feed
  • Has unrestricted access to lush pasture
  • Is provided forage that is higher quality than required

Most of your horse’s diet should be fibre-rich forage. The quality of your horse’s forage should also match their energy needs.

Mature horses at maintenance (not exercising) can maintain a healthy body condition with relatively low-quality grass hay. These horses may consume too many calories if provided high-quality forages, such as alfalfa or immature grass hay.

In comparison, horses with higher energy needs such as performance horses or lactating mares require higher-quality forages to maintain their body condition.

Mad About Horses
Join Dr. Chris Mortensen, PhD on an exciting adventure into the story of the horse and learn how we can make the world a better place for all equines.
Apple Podcasts Spotify Youtube
Mad Barn - Equine Nutrition Consultants

Breed & Genetics

Some equine breeds are more prone to obesity than others. These breeds are generally identified as easy-keepers or good-doers because they tend to gain weight easily and require less feed to maintain body condition. [10]

Cob-type horses and native pony breeds are at the greatest risk of obesity. Cobs are 13.6 times and ponies are 2.3 times more likely to be obese than lightweight breeds. [9]

Drafts and Quarter Horses have the next highest risk of being overweight, followed by Tennessee Walking Horses. Lightweight breeds such as Standardbreds, working Arabians and Thoroughbreds have a lower risk of obesity. [9][12][13]

Genetic Traits

Certain breeds are more susceptible to becoming obese because of their genetics. Several traits linked to obesity such as cresty neck and insulin resistance have a genetic component in horses.

These traits may be heritable (passed through family lines) and make certain breeds more susceptible to obesity and/or related consequences. [14][15]

For example, a study in Welsh ponies and Morgan horses prone to obesity found a higher proportion of genetic variants linked to inflammation and dysregulated sugar and fat metabolism. [16]

Evolutionary Adaptation

The predisposition to obesity may be an evolutionary adaptation to poor-quality forages found in the wild.

For example, native breeds in the UK are described as thrifty and can obtain sufficient energy from sparse, fibrous low-calorie grasses. [8][9]

When these breeds are fed improved high-quality forages, grains or commercial feeds they extract an abundance of calories, leading to weight gain.

Dominance

Horses considered to be more dominant within the social hierarchy of their herd are at greater risk of obesity. [12]

Dominant horses have better access to communal food sources, such as hay feeders, and may consume more calories than their less dominant counterparts.

Horses that are higher in the herd hierarchy are also given more space to graze when turned out in a group. In comparison, low-ranking horses spend more time and energy being chased away or moving around dominant horses and may have a higher stress level.

Age

Horses over 4 years old are at a greater risk of becoming overweight. [9]

Younger horses expend more energy growing and are more active than older horses, which could explain the difference in obesity risk between age groups. [9]

Historically, senior horses were likelier to be underweight than overweight. Horses over the age of 20 have more dental issues, exercise intolerance, and metabolic dysfunction that influences their ability to consume and expend calories. [17]

However, with modern management practices including regular deworming, dental and veterinary care, senior horses are better able to maintain a healthy body condition.

In recent surveys in North America, Europe, and Australia, senior horses are now almost equally likely to be overweight as underweight. [17]

Workload

Horses with a heavier workload have a lower risk of becoming overweight because of their increased energy expenditure. The more activity that your horse gets, the less likely they are to consume more calories than they are burning.

Horses ridden for pleasure have a lower risk of becoming overweight, and competition horses or horses in intense exercise routines have an even lower risk of obesity. [13]

Exercise also helps to improve insulin sensitivity, which contributes to healthy weight management. Seven days of light exercise improve insulin sensitivity in obese horses by as much as 60%. [36]

Management

The management of horses encompasses everything from housing to daily routine, feeding, socialization, training, veterinary care, and more.

Management factors can impact your horse’s obesity risk by affecting feed intake, nutrient utilization and energy expenditure.

Several survey studies have identified management risk factors for obesity. One study found that larger, professional horse-keeping facilities had fewer overweight horses. [11]

Larger facilities are likely more knowledgeable about feeding, exercising, and maintaining an ideal weight on horses. Weight maintenance may also be more important to professional facilities with horses used in shows or competitions.

Another survey found that horses living on farms raising non-equine livestock for meat were more likely to be overweight. [11] This may be attributed to different feeding practices.

Season

Horses have a higher risk of becoming overweight in the summer than in the winter. [11] In one study, the prevalence of obesity was 35% at the end of summer compared to 27% at the end of winter. [9]

This may be attributed to greater energy expenditure in the winter as horses burn calories through thermogenesis (heat production) to maintain a stable body temperature. [11]

It is quite normal for horses to lose body condition throughout the winter because there is less available forage and the horse’s maintenance energy requirements increase due to thermoregulation. [9]

Horses have greater forage availability in the warmer seasons, especially if they are turned out with unrestricted access to lush pastures. [9][11]

Sugar and starch levels in grass are highest in the spring when the grass is lush and growing quickly. This can contribute to weight gain in horses as summer approaches. [18]

Sugar and starch are non-structural carbohydrates (NSC), which are quickly digested in the foregut and provide the horse with rapidly available energy. In excess, non-structural carbohydrates contribute to weight gain.

Injury

Injured horses are prone to weight gain due to reduced activity and lower energy expenditure. [9]

If your horse is injured and put on stall rest or assigned a lower workload, consult with an equine nutritionist to adjust their feeding plan. Feed and pasture must be reduced in proportion to the decrease in exercise to prevent weight gain.

Negative Effects of Obesity in Horses

Horses become obese from consuming more energy than they expend. These extra calories contribute to fat accumulation and cause physiological changes that are generally detrimental to equine health.

Insulin Levels and Glucose Storage

When horses consume feed containing glucose (derived from starch or simple sugars), blood glucose levels rapidly rise. This triggers insulin secretion from the pancreas.

Insulin is a hormone that regulates blood sugar levels by promoting glucose uptake into cells. Insulin also tells the body to store glucose as glycogen or fat until the horse needs it for energy.

In some horses, prolonged intake of high carbohydrate feeds such as grains can lead to impaired insulin sensitivity. This means that the horse’s cells do not effectively respond to the presence of insulin. However, this is compensated for by an increase in glucose uptake by pathways that do not require insulin. [19]

Horses with low insulin sensitivity are described as insulin-resistant. Horses with insulin resistance take longer to clear glucose from the blood after a high carbohydrate load but usually have normal resting blood glucose. [19]

In response to hyperglycemia, the pancreas produces more insulin to remove extra glucose from the blood. Hyperinsulinemia or high insulin in the blood can contribute to complications such as laminitis. [19][20]

Altered Fat Metabolism

Horses and other mammals convert excess glucose into glycogen, which is made of long chains of glucose molecules. Glycogen is a rapidly available form of energy stored in the liver and skeletal muscle.

Glucose is also stored as fat molecules in adipose tissue. Adipose tissue can be found throughout the body but mainly under the skin (subcutaneous adipose) or around organs within the body (visceral fat).

The cells of adipose tissue are designed to store large amounts of fat. However, in obese animals, these cells might be unable to keep up with energy storage demands.

This can lead to spillover in which fat is no longer just stored in adipose tissue, but also gets stored in the liver and muscle. [21]

A blood test in an overweight or obese horse may reveal issues with fat metabolism in extreme cases such as:

  • High levels of triglycerides
  • High levels of cholesterol produced by the liver
  • Abnormal liver enzyme levels suggesting damage from fat accumulation in the liver

Systemic inflammation

As adipocytes (adipose tissue cells) expand in size to store more fat, they experience stress. This causes white blood cells to invade the fat to release proinflammatory molecules known as cytokines.

Cytokines are proteins including TNF-alpha, interleukin-1, interleukin-6, and interleukin-8. They are responsible for upregulating the immune system and inducing inflammation. [22]

The systemic inflammatory response is well documented in humans but the picture in horses is not as clear. In obese ponies, there was no difference in inflammatory cytokines between insulin sensitive and insulin resistant individuals. [38]

The first study looking at inflammatory cytokines, obesity and insulin resistance was done in mares. It looked at messenger RNA for three cytokines but actual circulating levels of only one, TNF-alpha.

TNF-alpha was positively correlated with insulin resistance only in mares over the age of 20. Because the study was done in the fall and on older horses there is a good chance the seasonal ACTH rise was complicating the picture. [39]

Leptin Resistance

Adipocytes (fat cells) produce signaling molecules known as adipokines, including the hormones adiponectin and leptin. [22]

Adiponectin is produced by adipocytes to promote insulin sensitivity and reduce inflammation. [12]

Leptin is a chemical messenger that helps to control appetite and promotes activity to maintain body condition. Leptin release is triggered by feed intake.

When your horse consumes calories, leptin sends a message to the brain that the body is in positive energy balance. This results in feelings of fullness and appetite suppression. [12]

Overweight horses can also get hyperleptinemia (high blood leptin) and become leptin resistant. Like insulin resistance, leptin resistance occurs when the brain stops responding to the presence of leptin and the body produces more of this hormone to compensate. [12][23]

In leptin-resistant horses, the absence of leptin signalling makes it even easier for overweight horses to gain excess weight. [12]

Blood Parameters

The following blood parameters often increase in overweight horses and affect or are affected by insulin sensitivity: [8][24][25][26]

  • Leptin
  • Insulin
  • Glucose (rarely)
  • Vitamin E (related to greater fat stores, not disease)
  • Cholesterol
  • Triglycerides

In contrast, adiponectin levels decreases in the blood as body mass increases. Adiponectin is the hormone produced by fat cells to combat inflammation and improve insulin sensitivity. [12]

Your veterinarian may assess some or all of these factors in your horse. Tracking these levels over time can help assess whether your horse’s metabolic health is improving or worsening.

Health Consequences of Obesity in Horses

Horses that carry excess body condition are at risk of many health conditions, including: [27]

  • Hepatic lipidosis
  • Osteochondrosis (developmental disease)
  • Impaired reproductive performance
  • Osteoarthritis (degenerative joint disease)
  • Lipidosis
  • Exercise intolerance
  • Heat intolerance
  • Hyperlipemia and dyslipidemia

Equine Metabolic Syndrome

Obesity is associated with Equine Metabolic Syndrome (EMS), a condition which is similar to Type 2 diabetes in humans. However, most horses with EMS do not progress to diabetes if fed correctly. It is important to note that association is not causation and there is a good chance the obesity is a result of EMS, not the cause.

EMS is characterized by several metabolic changes in the horse including rare hyperglycemia (high blood glucose), hyperinsulinemia (high blood insulin), and insulin resistance (the body is less responsive to insulin). [22][28]

General obesity may be associated with just one, multiple or none of these metabolic alterations. [22]

Horses with EMS are also likely to have excessive regional fat deposits in specific areas such as the crest of the neck or prepuce.

Metabolic syndrome also puts horses at high risk of developing laminitis. [28]

Laminitis

A body condition of 6 or higher puts horses at greater risk of laminitis and founder, likely because EMS is more prevalent in obese horses. [22]

The coffin (pedal) bone inside the horse’s hoof is connected to the hoof wall by soft tissue structures known as laminae.

In horses with laminitis, the laminae become weakened and elongated and in severe cases can break down. This causes the coffin bone to loosen from the hoof wall and rotate or drop lower in the hoof capsule (sinking). This puts pressure on the sole of the horse’s foot and causes severe pain. [28]

Pituitary Pars Intermedia Dysfunction (PPID)

Pituitary Pars Intermedia Dysfunction (sometimes referred to as Equine Cushing’s Disease) is a neurodegenerative condition that affects hormones produced by the intermediate lobe of the pituitary gland in the brain.

Symptoms include possible changes in behaviour, increased drinking and urination, impaired immune function, tendon and ligament problems, pendulous abdomen, cresty neck, muscle loss, and growth of a thick winter coat that is slow to shed out.

Horses with PPID may have elevated serum insulin levels. [#] However, even horses not genetically predisposed to metabolic syndrome can develop it with PPID.

As the condition progresses, weight loss and loss of muscle mass are common. In some horses with PPID, insulin resistance persists putting them at higher risk of laminitis. Cresty neck may also be observed in horses with PPID and insulin resistance. [29]

Hepatic Lipidosis

Overweight horses are at risk of hepatic lipidosis or fatty liver syndrome, a state in which fat accumulates in the liver and impedes liver function. [19]

The liver plays an important role in metabolism, detoxification and immune function. If excess fat accumulation impairs liver function, the horse can experience significant complications. [37]

Hepatic lipidosis secondary to hyperlipidemia syndrome most commonly affects ponies, donkeys, and miniature horses. [37] Animals with PPID and EMS sometimes show elevated liver enzymes suggestive of hepatic dysfunction.

Osteochondrosis

Rapid growth rate and over-conditioning are risk factors for developmental orthopedic disease (DOD). [21]

Osteochondrosis affects young, growing horses and is characterized by abnormal cartilage growth on the ends of bones where they form a joint with other bones. Severe cases of osteochondrosis can cause joint inflammation or even lameness in young horses.

Growing horses that are fed excess calories to promote fast growth are most at risk of becoming overweight and developing DODs. [30]

Reproduction

Obese horses may have fertility issues and reduced reproductive efficiency. Time between ovulations is longer in obese mares than in feed-restricted mares, resulting in fewer breeding opportunities.

The hormone progesterone remained elevated for longer than normal in overweight mares, suggesting that mechanisms for triggering ovulation are suppressed in obese mares. [31]

However, horses with a body condition score below 5 also have a reduced pregnancy rate and are less likely to maintain pregnancy to term. Keeping broodmares at the optimal weight of 5 or 6 is important for fertility and performance. [32]

Other Impacts of Obesity

Obesity in horses often coincides with elevated consumption of carbohydrates. High-carbohydrate diets can lead to gastrointestinal issues such as hindgut acidosis, altered microbiome, and increased gut permeability. [22]

Dramatic changes in gut function with grain overload has been linked to laminitis. [33]

Excess body fat also acts as an insulating layer and can interfere with thermoregulation – the horse’s ability to regulate their core temperature. This could lead to heat intolerance and poor performance in hot conditions.

The impact of obesity on thermoregulation has not been studied in horses and is poorly understood in most mammals. However, excess fat makes it more difficult for the body to dissipate heat in humans, putting overweight humans at higher risk of heat stress. [34]

Supporting Weight Loss in Obese Horses

If your horse is obese or overweight, work with your veterinarian and equine nutritionist to formulate a feeding and management plan to support healthy weight loss.

  • Start by assessing your horse’s body condition and current activity level to determine their energy requirements and how much weight they need to lose.
  • Evaluate your horse’s current feeding program to calculate how much digestible energy is being provided. Overweight horses need lower energy in their diet to create a calorie deficit.
  • Remove high-calorie commercial feeds, supplemental fat sources and concentrates such as grains in your horse’s diet. [12]
  • Choose lower-quality forages while ensuring the diet provides adequate protein. Submit a forage sample for analysis and work with a nutritionist to understand its composition.
  • If your hay has more than 10% ESC (sugar) plus starch consider soaking your hay.
  • Rinsed and soaked beet pulp is a suitable low-calorie carrier for supplements.
  • If removing grain and concentrates does not allow your horse to reach an ideal body condition, limit forage to 1.4-1.7% of the horse’s body weight per day. [12] A slow feeder or a hay net is recommended to extend your horse’s foraging time.
  • Avoid over-restricting feed intake, which is a risk factor for hyperlipidemia in ponies, donkeys and miniatures and can worsen insulin resistance in any horse.
  • Ensure that your horse’s vitamin and mineral requirements are still being met. Feed a concentrated vitamin and mineral supplement such as Mad Barn’s Omneity to support metabolic health and overall well-being.
  • If your horse is sound, implement an appropriate exercise routine to increase your horse’s energy expenditure and improve insulin sensitivity.

Regularly assess your horse’s body condition to monitor progress and determine whether further dietary changes are needed to reach your horse’s goal weight

For additional tips on feeding and managing obese horses, submit your horse’s information online to receive a free diet evaluation from our qualified equine nutritionists.

Is Your Horse's Diet Missing Anything?

Identify gaps in your horse's nutrition program to optimize their well-being.

References

  1. Rendle, D. et al. Equine obesity: current perspectives. UK-Vet Equine. 2018.
  2. Menzies-Gow N.J., Harris P.A., Elliott J. Prospective cohort study evaluating risk factors for the development of pasture-associated laminitis in the United Kingdom. Equine
  3. Harker, I.J. et al. The body condition score of leisure horses competing at an unaffiliated championship in the UK. J Equine Vet Sci. 2011.
  4. Ribeiro, R.M. et al. Insulin dysregulation in horses with induced obesity. Pesq Vet Bras. 2020.
  5. Morrison P.K. et al. Perceptions of obesity and management practices in a UK population of leisure-horse owners and managers. J Equine Vet Sci.
  6. Henneke, D.R. et al. Relationship between condition score, physical measurements and body fat percentage in mares. Equine Vet J. 1983. View Summary
  7. Dugdale A.H.A. et al. Body condition scoring as a predictor of body fat in horses and ponies. Vet J. 2012. View Summary
  8. Frank, N. et al. Physical characteristics, blood hormone concentrations, and plasma lipid concentrations in obese horses with insulin resistance. J Am Vet Med Assoc. 2006. View Summary
  9. Giles, S.L. et al. Obesity prevalence and associated risk factors in outdoor living domestic horses and ponies. PeerJ. 2014. View Summary
  10. Robin, C.A. et al. Prevalence of and risk factors for equine obesity in Great Britain based on owner-reported body condition scores. Equine Vet J. 2015. View Summary
  11. Hitchens, P.L. et al. Prevalence and risk factors for overweight horses at premises in Sweden assessed using official animal welfare control data. Acta Vet Scand. 2016. View Summary
  12. Durham, A.E. et al. ECEIM consensus statement on equine metabolic syndrome. J Vet Intern Med. 2019. View Summary
  13. Robin, C.A. et al. Prevalence and risk factors for owner- reported obesity in horses and ponies in Great Britain. Equine Vet J. 2013.
  14. Sanchez, M.J. et al. Prevalence, risk factors and genetic parameters of cresty neck in Pura Raza Español horses. Equine Vet J. 2016.
  15. Lewis, S.L. et al. Genomewide association study reveals a risk locus for equine metabolic syndrome in the Arabian horse. J Anim Sci. 2017. View Summary
  16. Norton, E.M. et al. Heritability of metabolic traits associated with equine metabolic syndrome in Welsh ponies and Morgan horses. Equine Vet J. 2018. View Summary
  17. Ralston, S.L. and Harris, P.A. Chapter 15 – Nutritional Considerations for Aged Horses. Equine Applied and Clinical Nutrition. 2013.
  18. Frank, N. et al. Equine metabolic syndrome. J Vet Intern Med. 2010. View Summary
  19. Hoffman, R.L. et al. Obesity and diet affect glucose dynamics and insulin sensitivity in Thoroughbred geldings. J Anim Sci. 2003. View Summary
  20. de Laat, M.A. et al. Hyperinsulinemic laminitis. Vet Clin North Am Equine Pract. 2010. View Summary
  21. Johnson, P.J. et al. Medical implications of obesity in horses—lessons for human obesity. J Diabetes Sci Technol. 2009. View Summary
  22. Geor, R.J. Metabolic predispositions to laminitis in horses and ponies: obesity, insulin resistance and metabolic syndromes. J Equine Vet Sci. 2008.
  23. Vick, M.M. et al. Relationships among inflammatory cytokines, obesity, and insulin sensitivity in the horse. J Anim Sci. 2007. View Summary
  24. Schedlbauer, C. et al. Alterations of serum vitamin E and vitamin A concentrations of ponies and horses during experimentally induced obesity. J Anim Physiol Anim Nutr (Berl). 2020. View Summary
  25. Ribeiro, R.M. et al. Changes in metabolic and physiological biomarkers in Mangalarga Marchador horses with induced obesity. Vet J. 2021. View Summary
  26. Johnston, L.W. et al. Association of NEFA composition with insulin sensitivity and beta cell function in the Prospective Metabolism and Islet Cell Evaluation (PROMISE) cohortChapman, S. Obesity and the health and welfare of the leisure horse. The veterinary Nurse. 2014
  27. Walshe, N. et al. A multiomic approach to investigate the effects of a weight loss program on the intestinal health of overweight horses. Front Vet Sci. 2021. View Summary
  28. Frank, N. Chapter 136 – Pituitary Pars Intermedia Dysfunction. Robinson’s Current Therapy in Equine Medicine (Seventh Edition). 2015.
  29. Bramlage, L.R. Physitis in the horse. Equine Veterinary Education. 2011.
  30. Vick, M.M. et al. Obesity is associated with altered metabolic and reproductive activity in the mare: effects of metformin on insulin sensitivity and reproductive cyclicityView Summary
  31. Henneke, D.R. et al. Body condition during pregnancy and lactation and reproductive efficiency of mares. Theriogenology. 1984.
  32. Milinovich, G.J. et al. Microbial Events in the Hindgut During Carbohydrate-induced Equine Laminitis. Vet Clin: Equine Pract. 2010.
  33. Speakman, J.R. Obesity and thermoregulation. Handb Clin Neurol. 2018.
  34. Geor, R.J. et al. Dietary management of obesity and insulin resistance: countering risk for laminitis. Vet Clin North Am Equine Pract. 2009. View Summary
  35. Powell DM. et al. Effect of short-term exercise training on insulin sensitivity in obese and lean mares. Equine Vet J Suppl. 2002. View Summary
  36. Foreman J. Disorders of the Liver in Horses. Merck Veterinary Manual. 2019.
  37. Ungru, K. et al. Effects of body weight reduction on blood adipokines and subcutaneous adipose tissue adipokine mRNA expression profiles in obese ponies. Vet Rec. 2012. View Summary
  38. Vick, M.M. et al. Relationships among inflammatory cytokines, obesity, and insulin sensitivity in the horse. J Anim Sci. 2007. View Summary